rec-compose4

W3C sample	WIRIS
[N/A]	$f g x$

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <apply>
    <ci>f</ci>
    <apply>
      <ci>g</ci>
      <ci>x</ci>
    </apply>
  </apply>
</math>

Author: MathML 2 Recommendation